The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit having layers containing an organic polymer called "kerogen", which upon heating decomposes to produce hydrocarbon liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale above ground, or processing the oil shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972 to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting a portion of the remaining formation to form a stationary, fragmented permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen containing retort inlet mixture into the retort as a gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the gaseous combustion zone feed into the combustion zone, the combustion zone is advanced through the retort. The combustion zone is maintained at a temperature lower than the fusion temperature of oil shale, which is about 2100.degree. F., to avoid plugging of the retort, and above about 1100.degree. F. for efficient recovery of hydrocarbon products from the oil shale.
The effluent gas from the combustion zone comprises combustion gas, carbon dioxide from mineral carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas is essentially free of free oxygen and contains constituents such as oxides of carbon, water vapor, nitrogen, and sulfurous compounds. It passes through the fragmented mass in the retort on the advancing side of the combustion zone to heat oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and to a residue of solid carbonaceous material.
As used herein, the term "processing gas" is used to indicate gas which serves to advance a processing zone such as a combustion zone, a retorting zone, or both a retorting zone and combustion zone, through the fragmented mass in an in situ oil shale retort, and includes, but is not limited to, an oxygen supplying gas introduced into a retort for advancing a combustion zone and retorting zone through a retort and a hot retorting gas which can be introduced into a retort or generated in a combustion zone in a retort for advancing a retorting zone through a retort.
The liquid products and gaseous products are cooled by cooler particles in the fragmented mass in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift or shaft. An effluent gas, referred to herein as off gas, containing combustion gas generated in the combustion zone, gaseous products including methane produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion process is also withdrawn from the bottom of the retort.
There are several reasons that it is desirable to know the locus of parts of the combustion and retorting processing zones as they advance through an in situ oil shale retort. One reason is that by knowing the locus of the combustion zone, steps can be taken to control the orientation or shape of the advancing side of the combustion zone. It is desirable to maintain a combustion zone which is flat and uniformly transverse and preferably uniformly normal to the direction of its advancement. If the combustion zone is skewed relative to its direction of advancement, there is more tendency for oxygen present in the combustion zone to oxidize hydrocarbon products produced in the retorting zone, thereby reducing hydrocarbon yield. In addition, with a skewed or warped combustion zone, more cracking of the hydrocarbon products can result. Monitoring the locus of parts of the combustion zone provides information for control of the advancement of the combustion zone to maintain it flat and uniformly perpendicular to the direction of its advancement to obtain high yield of hydrocarbon products.
Another reason that it can be desirable to monitor the locus of the combustion zone is to provide information so the composition of the combustion zone feed can be varied with variations in the kerogen content of oil shale being retorted. Formation containing oil shale includes horizontal strata or beds of varying kerogen content, including strata containing substantially no kerogen, and strata having a Fischer assay of 80 gallons of shale oil per ton of oil shale. If conbustion zone feed containing too high a concentration of oxygen is introduced into a region of the retort containing oil shale having a high kerogen content, oxidation of carbonaceous material in the oil shale can generate so much heat that fusion of the oil shale can result, thereby producing a region of the fragmented mass which cannot be penetrated by retorting gases.
Another reason for monitoring the locus of the combustion and retorting processing zones as they advance through the retort, is to monitor the performance of the retort to determine if sufficient shale oil is being produced for the amount of oil shale being retorted.
Also, by monitoring the locus of the combustion and retorting zones, it is possible to control the advancement of these two zones through the retort at an optimum rate. The rate of advancement of the combustion and retorting zones through the retort can be controlled by varying the flow rate and composition of the combustion zone feed. Knowledge of the locus of the combustion and retorting zones allows optimization of the rate of advancement to produce hydrocarbon products of the lowest cost possible with cognizance of the overall yield, fixed costs, and variable costs of producing the hydrocarbon products.
Thus, it is desirable to provide methods for monitoring advancement of combustion and retorting processing zones through an in situ oil shale retort.